1. Field of the Invention
The present invention generally is directed to means and methods for controlling electric motors. More specifically, the invention is directed to means and methods for controlling the torque developed by single phase induction motors thereby controlling their speed and/or direction.
2. Background of the Related Art
Single phase induction motors (SPMs) constitute the majority of motors used in home appliances today. SPIMs include permanent split capacitor (PSC) motors, split phase motors, capacitor start motors, capacitor run motors, permanent magnet synchronous motors, and shaded pole motors. As is known, these motors are inherently single speed motors and are used in various machines like clothes washers, clothes dryers, dishwashers, hermetic compressors, fans, pumps, draft inducers, et cetera It is also known that single phase induction motors inherently produce torque after they are running but they produce no starting torque.
There are several well known techniques for providing the necessary starting torque. For example, some prior art motors supply the first winding directly from the ac utility line and supply the second winding from the ac line through a reactive element, such as a capacitor, to generate the necessary voltage time displacement, known as a phase shift. Other prior art motors switch the reactive element out of the circuit as is done in a capacitor start motor, or left in the circuit to improve running performance as in a permanent split capacitor (PSC) motor.
SPIMs, generally are supplied power from an ac utility line at 50 or 60 Hz and at 120 or 230 volts. These motors typically have an output power rating of between 5 to about 1500 watts.
As mentioned above, these motors operate at constant speed when supplied from the ac utility line. When multiple speeds are needed, techniques like multiple windings with different number of poles, or tapped windings are used. In some applications, like fans, blowers, or pumps, the applied voltage to the motor is reduced in order to decrease the motor speed.
In applications where direction changing is required for a motor, a SPIM with only one winding energized full time is generally stopped or slowed to a speed where a start switch (e.g. centrifugal switch) energizes a start winding again, and then the motor is started in the other direction. Startng in the other direction requires a second start winding that is wound 180 electrical degrees out of phase with the first start winding or a switching system that reverses the polarity (connections) of the first start winding such that the flux produced by the second start winding will be 180 degrees out of phase with the flux produced by the first start winding. These motors can also be reversed under power by switching in the second winding before stopping or slowing the rotor; however, this is generally not done because of the higher reversing current (load point at reversal gives a very poor torque per ampere ratio).
A SPIM with both windings energized full time, such as a permanent split capacitor motor, is reversed by using a switching system that reverses the polarity (connectors) of one of the windings or changes the connection of the capacitor from being in series with one of the windings to being in series with the other winding. Reversals under power are more common with this type of motor as they are generally designed with a better torque per ampere ratio at reversal.
In industrial applications, three phase induction motors have been used instead of SPIMs due to the general availability of three phase power, as well as the higher output capabilities, higher torque production, higher efficiency and lower inrush currents of the three phase motors. Multiple speeds, torque and a means to reverse are typically provided in the motor's controller. However, in residential appliances, SPIMs are preferred due to the greater availability of single phase power.
Furthermore, the variable speed three phase motors, notably induction motors driven by hard switching three phase inverters as commonly used in industrial applications are not suitable for the home appliances because home appliance motor designs are driven by low cost, high efficiency, small size, and high production volume, which are not the main driving forces behind industrial motor controller designs.
The following patents disclose various means and methods for driving polyphase motors and/or SPIMs. A polyphase motor is designed to have at least two windings, where each winding is connected across a different set of ac voltage supply terminals. The ac voltages supplied for starting and running the polyphase motor are of the same frequency but differ in phase (more than one phase). A SPIM being different than a polyphase in that the SPIM is designed to have one or more windings that connect either directly on indirectly across a single set of ac voltage supply terminals for both starting and running.
U.S. Pat. No. 4,060,754 discloses an electronic motor that operates from a single phase power supply and has operating characteristics of a polyphase motor. One of the motor's windings is connected directly to a source of single phase power, the second winding is connected to an electronic waveform synthesizer that produces a stepped sinusoidal waveform that is shifted in phase appropriate to start and run the polyphase motor.
U.S. Pat. No. 4,401,933 discloses a SPIM control system that uses power line assisted starting and runs at higher Oman power line frequency from an electronically generated inverter supply. The motor's start winding is connected to a source of single phase power, the second winding is connected to an inverter that provides a phase shifted voltage. The motor stats and comes up to a minimum speed (around 50 percent of final operating speed), the start winding is then disconnected and the motor continues to accelerate to the final operating speed under the action of the run winding and the inverter. The final speed is set by the output frequency of the inverter This patent also discloses a sense winding to sense the current in the start winding, and a braking circuit to stop the motor.
U.S. Pat. No. 4,117,364 discloses a variable frequency voltage waveform synthesizer driving a rotating electric machine similar to the above mentioned patents. The synthesizer provides a stepped waveform output with the widths of the steps being controllable as to affect power transfer of the waveform as well as the harmonic content therein.
Thus far, control systems for SPIMs that convert fixed frequency AC power to DC power and then invert the DC power to a voltage, of variable amplitude, frequency, and phase angle, to control the speed of a single phase induction motor do not optimize the control for the torque per ampere ratio of the motor over the full range of operating conditions. Very often the control will be optimized with a focus on one operating point or condition.
Generally this is a specific speed and torque, a stating condition, or loads driven at speeds above synchronous speed.